The invention relates to a process and installation for heating coal to temperatures between about 100.degree. and 500.degree. C., preferably to about 200.degree. C.
In this process the coal is subjected first to drying and then to heating. If this process is carried out in two or more stages a high degree of thermal efficiency is reached if the heat carrier gases are used first for heating and then for drying the coal, whereupon they may be at least partially passed into the atmosphere. The heat for drying the coal in this process is generally obtained from the relatively less hot heat carrier gases upon raising of the temperature to about 80.degree. to 100.degree. C. while the very hot heat carrier gases leaving the combustion chamber at about 600.degree. C. result in a heating up of the coking coal to about 200.degree. C.
The heat carrier gases which are exhausted from the drying stage usually undergo a cooling down of the temperature to a point where in case of gases having a substantial sulfur contents the sulfuric acid dewpoint is usually reached and passed. This is particularly critical where the water contents of the feed product changes quickly and thus suddenly rises above the starting value. The same is true for variations in the amount of throughput. Once the sulfuric acid dewpoint or even the water vapor dewpoint is reached, substantial difficulties for the various parts of the installation may ensue, for instance for the dust precipitator, the blower, the ducts, the measuring and control devices, etc., and this both as to their function and as to the possibility of corrosion.
The heating of coking coal by means of hot gases obtained from a combustion chamber to temperatures above 150.degree. in two stages is known, the first stage serving as the drying zone and the second stage as the heating zone. The exhaust gases of the heating stage in this case are then used for the drying stage. One has also provided an exhaust gas duct between the heater and dryer which leads into the exhaust coming from the drying stage at a place immediately before the dust precipitator. This arrangement was intended to avoid the corrosion of the fine dust precipitator. It has, however, the disadvantage that all parts of the installation prior to the precipitator such as ducts, blowers, cyclone separators, elbow bends leading to the cyclones, recycling ducts for the hot vapors are still subject to corrosion. If the gas from the heater stage obtained from the cyclone separator were passed into the exhaust gas prior also to the blower, problems would occur because of excessive gas volumes in view of the low enthalpy. The blower would have to handle larger amounts and the gas ducts would have to be designed to excessively large dimensions.
It is therefore an object of the present invention to avoid these shortcomings of prior art devices and to prevent damage, in particular corrosion, to the parts through which the exhaust gas from the drying stage passes, in particular the fine dust precipitator.